Tafel constant table

Table 21.12 Tafel constants for hydrogen evolution from aqueous solution ...

Comparison of the anodic and cathodic Tafel constants shows that when a = 0.5, a = — a -, b = — b. Tables 3.2 and 3.3 list values of the Tafel constants for cathodic hydrogen evolution at T= 20 it 2 ° C on different metals and the effect of electrode materials and solution composition on oxygen overpotential [21]. The Tafel equation has been confirmed for numerous cathodic and anodic reactions, and its use is illustrated in the examples and case studies that follow. 

Estimate the effect of 1% impurity of platinum on the corrosion rate of zinc in acidic and alkaline solutions. Use the values of Tafel constants for the hydrogen evolution on the respective metals from Table E6.1. 

Table E6.1 Tafel Constants for the Hydrogen Evolution on the Respective Metals...

Table E6.2 Tafel Constants for Each of the Metals in the Alloy...

Table 4.1 Exchange current densities io, cathodic Tafel constants and overvoltages at i = 1 mA/cm [4.6], (Reproduced from Uhlig HH. Corrosion and Corrosion Control. New York John Wiley and Sons, 1971.)...

Table 31.2 indicates that the desired range of Tafel constant ratios (/3c//8a) for samples exhibiting good protection in NaCl is similar to that in HCl (Table 31.1). Samples that exhibited Tafel constant ratios around 0.5 in NaCI seem to provide good protection. In HCl, protection seems to be provided by strong cathodic polarization for hydrogen evolution at early stages of immersion. The Tafel constant ratio for best corrosion protection is close to I at intermediate immersion time. At the end of immersion, a low Tafel constant ratio indi-...

Table 31.2 Tafel Constant Ratios (/8c//3a) for D,e/cp/s Samples in 3.59f NaCl...

Table 5.5 Constants a and b of the Tafel equation and the probable mechanism of the hydrogen evolution reaction at various electrodes with H30+ as electroactive species (aH3o+ ) (According to L. I. Krishtalik)... 

Deviation of 60 mV/decade can be seen in Table 5.3 under different conditions. In addition to the potential distribution in the two double layers, there are two other possible causes for the deviations. The first is possible potential drops in other parts of the electrical circuit, e.g., in the electrolyte and semiconductor. The second possibility is the change of effective surface area due to the formation of a porous silicon layer during the course of i-V curve measurement. In addition, if the reaction is controlled by a process involving the Helmholtz layer, the apparent Tafel slope may be smaller than the 60 mV/decade as would be expected from the formula, B = kTI23anq, because the effective dissolution valence n is not a constant with respect to potential but varies from 2 to 3 in the exponential region. 

THE PROBLEM The polarization data in Table 3.3 have been obtained for the hydrogen evolution reaction on a rotating disk electrode. Calculate the constants of the resulting Tafel-type equation. 

Explanation 2 was proposed in Ref 31 for a coupled Vohner-Tafel path and is valid at constant current density, assumed to be due mainly to ihe combination rate (see Table 2), However, a lowering of the rate constant for siuface-bulk transfer cannot by itself explain why H permeation is increased under potentiostatic conditions [113], At fixed potential, the weakening of the bond strength,... 

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